British rock singer Ian Watkins, the former frontman of the band Lostprophets, has pleaded guilty to the attempted rape of a baby and a string of other child sex offences.

Watkins, whose band sold millions of albums around the world and toured Australia last year with the Soundwave festival, plotted the abuse of two children with their own mothers in a series of text and internet messages.

The 36-year-old spoke of his desire to intoxicate one of his victims by blowing crystal meth smoke into the child's face.

He made a last-minute change of plea before his trial started at Cardiff Crown Court in Wales on Tuesday (local time), sparing the jury from seeing what the judge called "extremely graphic and distressing" video footage.

He will be sentenced on December 18.

"Ian Watkins was the lead singer of a successful band called the Lostprophets," prosecution lawyer Christopher Clee told the court.

"He was also a determined and committed paedophile engaged in serious sex offences involving two babies."

Watkins plotted the abuse of a one-year-old child in messages with the mother, including one in which he said he wanted a "summer of filthy child porn" and to take things "to the next level".

He then recorded camcorder footage of himself performing a sex act on the child during a drug-fuelled binge, after arranging to meet the child's mother in a London hotel room.

Detectives found the footage uploaded to an online "cloud" storage facility.

Watkins then persuaded another female fan over the Skype network to abuse her own child.

The split screen footage shows him referring to the woman and her baby as his "slave duo".

They also discuss making the child have sex with animals and take drugs as it grows up.

Police raided Watkins's house and found films of him performing sex acts with under-age girl fans.

A laptop seized from his home had to be cracked by the British government's electronic eavesdropping system GCHQ, the court heard.

Watkins was originally charged with rape but insisted actual sex with the child did not happen.

He pleaded guilty to 11 charges on Tuesday, including two of attempted baby rape and others involving possessing and making images of sexual abuse and conspiracy to rape.